Various devices can be placed in a well traversing a hydrocarbon bearing subterranean formation. Some devices emit magnetic fields, such as static magnetic fields and radio frequency (RF) magnetic fields, into the formation to measure properties of the formation. Fluids in the wellbore can have properties such as electrical conductivity that can negatively affect transmission of magnetic fields into the formation. For example, wellbore fluids can diffuse a transmitted signal and cause losses in magnetic fields communicated into the wellbore. Losses in magnetic fields communicated into the wellbore can reduce efficiency and accuracy of downhole devices.